I just updated my Joomla dev page to Joomla 6.0.2. When editing an article and the t4 Plugin (2.5.4) is activated, i get this error stack:
When i deactivate the plugin, i can access the article without any problems.
An error has occurred.

0 Joomla\Registry\Format\Json::stringToObject(): Argument #1 ($data) must be of type string, null given, called in /var/www/html/libraries/vendor/joomla/registry/src/Registry.php on line 335

Call Stack

Function Location

1 () JROOT/libraries/vendor/joomla/registry/src/Format/Json.php:53
2 Joomla\Registry\Format\Json->stringToObject() JROOT/libraries/vendor/joomla/registry/src/Registry.php:335
3 Joomla\Registry\Registry->loadString() JROOT/plugins/system/t4/src/t4/Helper/ExtraField.php:211
4 T4\Helper\ExtraField::contentExtraFields() JROOT/plugins/system/t4/src/t4/Helper/ExtraField.php:104
5 T4\Helper\ExtraField::extendForm() JROOT/plugins/system/t4/src/t4/T4.php:174
6 T4\T4->contentPrepareForm() JROOT/plugins/system/t4/t4.php:152
7 PlgSystemT4->onContentPrepareForm() JROOT/libraries/src/Plugin/CMSPlugin.php:386
8 Joomla\CMS\Plugin\CMSPlugin->Joomla\CMS\Plugin{closure}() JROOT/libraries/vendor/joomla/event/src/Dispatcher.php:454
9 Joomla\Event\Dispatcher->dispatch() JROOT/libraries/src/MVC/Model/FormBehaviorTrait.php:196
10 Joomla\CMS\MVC\Model\FormModel->preprocessForm() JROOT/administrator/components/com_content/src/Model/ArticleModel.php:1047
11 Joomla\Component\Content\Administrator\Model\ArticleModel->preprocessForm() JROOT/libraries/src/MVC/Model/FormBehaviorTrait.php:115
12 Joomla\CMS\MVC\Model\FormModel->loadForm() JROOT/administrator/components/com_content/src/Model/ArticleModel.php:468
13 Joomla\Component\Content\Administrator\Model\ArticleModel->getForm() JROOT/libraries/src/MVC/View/FormView.php:197
14 Joomla\CMS\MVC\View\FormView->initializeView() JROOT/administrator/components/com_content/src/View/Article/HtmlView.php:84
15 Joomla\Component\Content\Administrator\View\Article\HtmlView->initializeView() JROOT/libraries/src/MVC/View/FormView.php:177
16 Joomla\CMS\MVC\View\FormView->display() JROOT/libraries/src/MVC/Controller/BaseController.php:697
17 Joomla\CMS\MVC\Controller\BaseController->display() JROOT/administrator/components/com_content/src/Controller/DisplayController.php:70
18 Joomla\Component\Content\Administrator\Controller\DisplayController->display() JROOT/libraries/src/MVC/Controller/BaseController.php:730
19 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/libraries/src/Dispatcher/ComponentDispatcher.php:143
20 Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:361
21 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:150
22 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:206
23 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:320
24 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/includes/app.php:58
25 require_once() JROOT/administrator/index.php:32